While the trooper is talking to him, he keeps swatting at a swarm of flies around him.
The farmer says “Having trouble with them circle flies, officer?”
The trooper says “Yeah– why do call them circle flies?”
The farmer says “Well they’re usually seen circling round the back end of horses.”
The trooper asks “Are you calling me a horse's ass?”
The farmer says “Oh no officer, I got too much respect for the police to do that. Hard to fool them circle flies though.”